The menstrual cycle derives from the Latin word for month and refers to the roughly one month (28 day cycle) that a woman goes through from the time she enters puberty until she loses her cycle at menopause. a normal menstrual cycle may be anywhere from 24-32 days in duration (quite in fact, while most women report a 28 day cycle, few actually have a 28 day cycle). The primary purpose of the menstrual cycle is to prepare for the potential of pregnancy and most of the hormonal changes and their effects are aimed at this goal. When the menstrual cycle is functioning normally, this is called eumenorrhea ("eu" means well or good). By convention, Day 1 of the cycle occurs at the onset of menstruation (i.e. her period), the bleeding that occurs as the uterine lining (thickened in preparation for implantation of the egg) is expelled. This typically lasts 3-5 days. From this point, the remainder of the cycle is divided into two distinct phases.
The Follicular Phase
The first half of the cycle is termed the follicular phase and within a 28 day cycle will typically last 14 days. Strictly speaking this can be divided up into an early, mid-, and late follicular phase of roughly 3-5 days each but I'll only use early- and late follicular in this blog with the split happening halfway through (i.e. day 7). In research, women are frequently measured during the mid-follicular phase to get a more or less average indication of what is going on physiologically. Under certain conditions, the follicular phase can lengthen as well. During this phase, the follicle (hence the name) develops due to the effects of Follicle Stimulating Hormone (FSH). Technically, multiple follicles develop but only one releases an egg.
Hormonally, progesterone remains very low during the follicular phase. Estrogen starts at a low level, shows a gradual increase leading up to a large surge in the final few days of the cycle. Along with this comes a surge of LH which causes the follicle to burst, releasing an egg into the fallopian tubes and this marks ovulation. The surge in estrogen also causes a thickening of vaginal mucus, which makes the vagina less acidic and more hospitable to sperm. Women have often used this thickening as an indication of their fertility. Finally, the endometrium thickens in preparation for implantation of the egg.
The Luteal Phase
The release of the egg on day 14 (the halfway point of the cycle) is termed ovulation and this marks both the middle of the menstrual cycle along with the end of the follicular phase (testosterone spikes briefly at this stage as well). At this point, a woman enters the luteal phase which can again be subdivided into early-, mid- and late phases. As with the follicular phase, I'll only use early- and late luteal phase in this blog. As in the follicular phase, researchers often measure women in the mid-luteal phase for consistency and to obtain an average response in terms of physiology.
During this phase, the burst follicle which released the egg develops into a structure called the corpus luteum (hence the name luteal phase) which produces the hormone progesterone which further prepares the endometrium to receive and nourish a fertilized egg. Over the first half of the luteal phase both progesterone and estrogen increase gradually reaching a peak at mid-cycle. Progesterone levels are higher than those of estrogen, which only reaches about half of the level seen during the peak of the follicular phase. Body temperature also increases slightly (about 0.2°C or 0.4°F) after ovulation and basal body temperature (BBT) can be used not only to tell when ovulation has occurred when pregnancy is the goal but is can also be used to tell when the follicular phase has ended and the luteal phase has begun. With the increase in body temperature comes an increase in metabolic rate which I will discuss in later blogs.
Late Luteal Phase
In the late luteal phase , progesterone and estrogen start to drop again and this is the when Premenstrual Syndrome (PMS), if present, typically occurs. PMS can be marked by an enormous number of symptoms including cramping (as a woman's body prepares to shed the uterine lining) mood swings, low energy, depression, breast tenderness and others (this is called dysmenorrhea). In extreme cases, women may experience debilitating pain from cramps, depression, anxiety or suicidal thoughts (often requiring medication) and this is referred to as Premenstrual Dysphoric Disorder (PMDD).
Depending on the source in question, PMS is reported to occur in roughly 30-40% of women with clinical relevant PMS occurring in 20% and PMDD occurring in 5-10% of women. Cramps per se are reported in 45-95% of women with 3-33% of women being physically incapacitated due to them. While cramping is often thought to occur primarily during the late luteal phase, it is common for it to continue through menstruation as the uterine lining is shed.
Summarizing the Cycle
Below is a graphic of a typical menstrual cycle , just showing the relative changes that occur in estrogen (black line), progesterone (gray line), testosterone (bottom black line) and body temperature (at the very top). The table also shows when menstruation, ovulation and PMS generally occur and how the different phases are named and divided.
As we go forward we are going to mainly focus on the effects of estrogen and progesterone, so for that reason I have deliberately left out two fairly important hormones which are Follicle Stimulating Hormone (FSH) and Leutinizing Hormone (LH). FSH is primarily involved with the development of the follicle itself while LH controls estrogen production, ovulation, implantation of the egg and development of the corpus luteum. I mention them here as the disruption of LH release patterns can occur under a variety of stressful conditions and this is what fundamentally leads to the menstrual cycle becoming disrupted.
Menstruation occurs from roughly day 1 to 5 before estrogen starts to climb slowly during the follicular phase of the cycle, surging in the final few days before dropping rapidly when ovulation occurs. Into the luteal phase, both estrogen and progesterone show a slow increase during the luteal phase hitting a peak at the middle of the cycle before gradually decreasing over the second half. If occurs at all, PMS or PMDD will occur in the last 4-7 days (roughly) before menstruation occurs and the cycle starts over.
As I discussed in the previous blog, the above dynamics should help to illustrate just how much more complex women are compared to men. Women have two primary hormones that are increasing, decreasing and criss-crossing and, as you'll see, interacting in complex and fascinating way. In contrast, a man's hormonal profile would be a more or less straight line for testosterone across every day of the month with not much else changing significantly. Those gender differences are present even before considering the hormonal modifiers that I will discuss in later blogs.
The above information is taken from the The Woman's Book by Lyle Mcdonald with Eric Elms